Possible linkage between the long-eccentricity marine carbon cycle and the deep-Pacific circulation: Western equatorial Pacific benthic foraminifera evidences of the last 4 Ma

The ~400-kyr period of marine carbon isotope records, in concert with the long eccentricity cycle, is a meaningful signature of the global carbon cycle, and this period was prolonged to ~500 kyr in the last ~1.6 Ma. The cause of the elongation of the long-eccentricity carbon cycle at ~1.6 Ma has not been fully understood, partly due to the conundrum of its relationship to changes in the circulation of deep-water masses. In this study, we present the benthic foraminifera δ13C and δ18O and the relative abundances of selected benthic foraminifera, from the upper ~80 m of Site U1489 of IODP Expedition 363. First, we established an age model for our samples, covering the last 4 Ma, based on the correlation of benthic foraminifera δ18O to the LR04 global stack as well as ship-board magneto-stratigraphy. Then we compared the δ18O and δ13C between Cibicidoides and Uvigerina to reconfirm the inter-genus isotopic adjustments. The coarse fraction, benthic foraminifera δ13C and the abundances of selected benthic species/genera show detectable 400–500 kyr cycles, with minimal coarse fraction and maximal δ13C around eccentricity minima. The variations in the relative abundances of selected benthic foraminifera, especially Favocassidulina favus, and the comparisons among deep-Pacific δ13C records, reveal a shift to a more sluggish mode of Pacific deep-water circulation occurring coincidently at ~1.6 Ma. Therefore, it may be implied that the lengthening of the long-term δ13C cycle at ~1.6 Ma is linked to a deep Pacific circulation change that prolonged the cycling of carbon in the ocean.